The invention relates to the technological field of structures used for preventing rechargeable electrical batteries from causing damage to their surroundings.
Misuse, such as overcharging, of a battery may result in generation of gas inside the battery and, hence, an increase in the internal pressure of the battery. The pressure may cause corrosive substances to spread outside the battery, resulting in damage to nearby circuits and equipment. In addition, misuse of the battery may result in a relatively dramatic build-up of thermal energy in the battery if electrical energy is no longer chemically absorbed. Accumulation of heat may also cause damage to structures around the battery.
Misuse of a battery is usually prevented by means of a safety circuit in the charging device: The battery voltage is monitored, and when it has reached a predetermined level corresponding to full charge, the charging current is electronically switched off. The charging current is not switched back on until the battery voltage has dropped to a second level considerably lower than the first one. Such an external safety circuit by itself provides inadequate protection. The charging device may become faulty so that the protection will not work. Furthermore, the battery may be connected to a charging device of an incorrect type, or even the wrong way around by its terminals. Safe batteries therefore have got internal protection in case of misuse.
From the prior art it is known internal battery protection methods as follows:
Use of internal protective components: A positive temperature coefficient (PTC) thermal resistor is placed in the battery circuitry. When the charging current is too large or the battery heats up because of chemical reasons the resistance of the thermal resistor rises, causing the charging current to drop. A disadvantage with the use of a thermal resistor is the limited protection it offers: A slight overcharging state may persist for a long time. In addition, a thermal resistor increases energy consumption during normal operation of the battery. Similarly, a thermal fuse can be placed in the battery circuitry. As the temperature of the battery reaches a certain limit, the thermal fuse disconnects the charging circuit. A disadvantage with the use of a thermal fuse is the energy consumption caused by it during normal operation of the battery.
Use of internal protective circuits: An electronic circuit is placed in the battery, sensing the battery current and voltage. If during the charging of the battery the behavior of the current or voltage deviates from the normal, said protective circuit disconnects the charging current. A disadvantage with the use of a protective circuit is that it increases the battery manufacturing costs. An additional disadvantage in this case, too, is that the energy consumption increases during normal operation of the battery.
Use of gas hole or valve: A small hole is made on a battery cell or a valve is placed in it such that gas possibly generated inside the battery can exit the battery. This eliminates a considerable increase in pressure and spreading of chemicals outside the battery in cases of gross misuse. There is, however, the disadvantage of heat accumulation in the battery and structures around it if the overcharging state is prolonged.
Mechanical disconnection of the charging circuit: FIG. 1 shows a breakable construction. On one side of a battery 10 there is a foil 11 made of a relatively thin conductive material. The battery charging circuit comprises a thin conductor 12 attached to the foil 11. The battery is hermetically sealed. If the battery is overcharged, the pressure of the gas generated distends the foil 11 as shown in FIG. 1c. As the pressure exceeds a certain threshold the conductor 12 breaks in two at point B and the charging current is disconnected. A disadvantage of the method is that it is only applicable to batteries which apart from said foil have a hard shell.
An object of the invention is to reduce said disadvantages related to the prior art. The protection method according to the invention is characterized by what is expressed in independent claim. Preferred embodiments of the invention are presented in the dependent claims.
The basic idea of the invention is as follows: A battery is manufactured such that pressure generated inside it will cause at least one of the battery""s conductors to tear in two, thus interrupting the flow of current through said conductor. To that end, the battery shell is made at least partly of a flexible material and is hermetically sealed. Said conductors are thin strips and their strength, or at least the strength of one of them, is reduced at one point e.g. by cutting a slit on a side of the conductor. The conductive strip is attached at one side of said point to the upper portion of the battery shell and at the other side to the lower portion of the battery shell. If pressure is generated inside the battery because of misuse, the pressure will distend the battery shell and the upper and lower portions of the shell will tear the conductive strip in two. A relatively small amount of force will suffice to accomplish that because the xe2x80x9ctearingxe2x80x9d has been started in advance by making said slit.
An advantage of the invention is that an increase in the internal pressure of a battery will not result in the spreading of corrosive chemicals outside the apparatus. Another advantage of the invention is that the charging of a battery is stopped before the battery will heat up its surroundings possibly to a damaging extent. A further advantage of the invention is that protection according to the invention will not cause extra energy consumption during normal operation of the battery. A yet further advantage of the invention is that it finds particular utility in batteries having a flexible construction; say, lithium polymer batteries, which are becoming more popular and for which there is no effective protection at the moment. Yet another advantage of the invention is that the costs of the arrangement according to the invention are relatively small. The costs are brought down among other things by the fact that the invention will make protective components redundant.